Tag axles are known in the art and are used as an expedient for reducing the per-axle load in a vehicle, Generally, when a vehicle is loaded, whether it be a dump truck, concrete mixer truck, or other load-carrying vehicle, the axle weight can be increased beyond that permissible for over-the-highway travel. This can result in damage to the highway and the truck and can create a hazard to the operator of an overloaded vehicle. A tag axle can be used to further distribute the vehicle and payload weight. The tag axle is loaded to relieve the other axles, and to that extent the load is relieved on the remaining axles, the per-axle weight is brought within that permitted by the various state laws for over-the-highway travel.
Unfortunately, tag axles in present use tend to be cumbersome and ineffective. Existing tag axles are mounted in such a way that when the wheels are moved downwardly into ground engagements, the limits of vertical movement of the tag axle are so limited that, in many cases, when the vehicle is moving under uneven terrain the tag axle bottoms out on the frame, and weight is lifted off the rear axles and tandem axles and transferred excessively onto the tag axle causing the tag axle wheels to dig into the ground. This is called "ploughing", and is an inherent result of many of the presently used tag axles because there is insufficient vertical movement for the tag axle before bottoming out on the frame.
Moreover, the air brakes and other associated power equipment for operating the air brakes are mounted in such a way on the tag axle as to interfere with normal and expected operation of the tag axle. In supporting the tag axle so that it can move vertically to cushion the load at the rear of the vehicle it is common present practice to use the large, weight-contributing structures which tend to limit normal operation of the tag axle.
In the present invention the purpose is to eliminate the previously used constructions which consisted of large centering guides, cross braces and blocks for guiding the wheels, as well as the inapt location of the air brake power actuation equipment, and substitute in its place a much simpler, cleaner construction in which the tag axle is supported and guided laterally by means of elevated guide means located one at each side of the frame and having rollers which engage in tracks or other bearing structures. In this way, the primary object of the invention is achieved, which is to permit minimized unsprung tag axle weight and a construction that avoids ploughing, i.e., digging into the ground when the vehicle is moved over irregular terrain and in which the components are more protected and less obstructive to normal vertical movement of the tag axle.
However, the means for guiding the wheels is greatly improved in that the wheels are permitted to move freely in a vertical sense, and one wheel can even move in a limited vertical sense relative to the other, to provide for normal encountering with chuck holes, potholes, and the like. At the same time, the wheels are prevented from moving laterally back and forth sidewise of the frame, and one wheel is prevented from advancing relative to the other along the length of the frame. Thus, the tag axle is prevented from pivoting about a vertical axis but can move perpendicularly, to allow for normal cushioning of the vehicle. But lateral movement of the wheels, i.e. movement horizontally to the frame, either side to side at the frame, or front to back of the frame, is totally prevented.
Each of the wheels includes a fender, and the fenders are braced for vertical movement with the wheel, the bracing being accomplished also through the tag axle.
It is possible, therefore, with existing vehicle structure, to take what is a commonly available tag axle. invert its normal position, weld to the existing frame a downwardly and rearwardly projected leverage system for supporting the tag axle, and then suspend the tag axle in a normally retracted position by means of springs located one on each side of the rails of the frame. Each side of the axle is then actuated by an associated air bag to effect downward movement of the wheels (rotatably supported on the tag axle) into ground engagement and thereby to distribute the vehicle load.
Other objects and features of the present invention will beome apparent from a consideration of the following description, which proceeds with reference to the accompanying drawings.